The present application is based on Japanese Patent Application no. 2000-097757, filed on Mar. 31, 2000, which is hereby incorporated by reference in its entirety.
1. Field of the Invention
This invention relates to a pulse tube refrigerator.
2. Description of the Background
In recent years, various structures for a pulse tube refrigerator are proposed. One is a four valve type pulse tube refrigerator is shown in FIG. 11. In FIG. 11 a four valve type pulse tube refrigerator 201 includes a compressor 10, a first high pressure on-off valve 21 and a second high pressure on-off valve 23 connected with a high pressure outlet port 10a of the compressor 10, a first low pressure on-off valve 22 and a second low pressure on-off valve 24 connected with a low pressure inlet port 10b of the compressor 10. A cryocooler 30 includes a regenerator 31, a cold head 32, a pulse tube 33 and a radiator 34 arranged in series in line. A hot end 31b of the regenerator 31 is connected with the first high pressure on-off valve 21 and the first low pressure on-off valve 22. A hot end 33b of the pulse tube 33 is connected with the second high pressure on-off valve 23 and the second low pressure on-off valve 24. Since the high pressure operating gas flows not only from the hot end 31b of the regenerator 31 but also from the hot end 33b of the pulse tube 33, the displacement of the operating gas in the pulse tube 33 is restricted, and the heat invasion from the hot end 33b of the pulse tube 33 into the cold head 32, which increases in accordance with the increase of the displacement of the operating gas in the pulse tube 33, can be restricted. Accordingly, a refrigeration efficiency is improved in comparison to a orifice buffer type pulse tube refrigerator.
The above four valve type pulse tube refrigerator with a high refrigerator efficiency still has a drawback: a generation of an unnecessary fluid return (DC flow). Since the hot end 33b of the pulse tube 33 is connected with the second high pressure on-off valve 23 and the second low pressure on-off valve 24, and the hot end 31b of the regenerator 31 is connected with the first high pressure on-off valve 21 and the first low pressure on-off valve 22, the compressor 10 and the cryocooler 30 form a closed circuit through each on-off valve. As a consequence, the operating gas circulates in the closed circuit independently of the cooling cycle. Due to the operating gas, the heat of the relativcly high temperature portion is transmitted into the cryocooler 30, and the refrigeration efficiency is decreased.
DC flow (Direct Current flow) is of two types, according to the direction of the flow. One of the DC flows is from the high pressure outlet port 10a of the compressor 10 through the first high pressure on-off valve 21 into the cryocooler 30 from the regenerator 31 side, and further from the hot end 33b of the pulse tube 33 through the second low pressure on-off valve 24, and returns to the low pressure inlet port 10b of the compressor 10. The other DC flows is from the high pressure outlet port 10a of the compressor 10 through the second high pressure on-off valve 23 into the cryocooler 30 from the pulse tube 33 side, and further from the hot end 31b of the regenerator 31 through the first low pressure on-off valve 22, and returns to the low pressure inlet port 10b of the compressor 10. The flow direction is determined depending on the operating condition of the pulse tube refrigerator. Both flows cause a decrease of the refrigeration efficiency due to the heat conduction by the DC flow. As a consequence, even in the four valve type pulse tube refrigerator, the improvement of the refrigeration efficiency is limited.
To solve the above explained drawbacks of the DC flow, an improved four valve type pulse tube refrigerator as shown in FIG. 12 has been proposed. A pulse tube refrigerator 202 includes the structure of the four valve type pulse tube refrigerator 201 as shown in FIG. 11, and also includes a fluid shield 40 connected with the pulse tube 33 (the radiator 34) at one end and connected with the second high pressure on-off valve 23 and the second low pressure on-off valve 24 at the other end. The fluid shield 40 is provided with a cylinder member 41 and a piston 42 slidably disposed in the cylinder member 41. The piston 42, and a piston ring 43 attached on the outer periphery of the piston 42, separate the interior of the cylinder member 41 into a first space 46 connected with the inner space of the pulse tube 33 and a second space 47 connected with the second high pressure on-off valve 23 and the second low pressure on-off valve 24. According to this structure, generation of the DC flow is interrupted by the piston 42 and the piston ring 43. Since the heat is not conducted by the DC flow, the refrigeration efficiency can be improved.
Even the above explained improved four valve type pulse tube refrigerator has a problem relative to refrigeration efficiency: an on-off valve loss. In the pulse tube refrigerator 202 shown in FIG. 12, before each valve is opened, a maximum pressure difference is generated in the spaces of both sides of the on-off valves. For instance, the second high pressure on-off valve 23 is positioned between the second space 47 and the high pressure space of the high pressure outlet port 10a side of the compressor 10. Immediately before the second high pressure on-off valve 23 is open, the second space 47 is under a minimum pressure condition. The second low pressure on-off valve 24 is positioned between the second space 47 and the low pressure space of the low pressure inlet port 10b side of the compressor 10. Immediately before the second low pressure on-off valve is opened, the second spaces 47 is under a maximum pressure condition. If each valve opens in this condition, energy loss is generated by the momentary occurrence of a no pressure differential condition. The larger the pressure difference is, the higher the energy loss becomes. Therefore, even in the improved four valve type pulse tube refrigerator, the improvement of the refrigeration efficiency is still limited.
It is, therefore, an object of the present invention to overcome the above drawbacks of the conventional refrigerator.
It is another object of the present invention to improve refrigeration efficiency by decreasing an on-off valve loss in an improved four valve typed pulse tube refrigerator.
In order to achieve the above and other objects, the pulse tube refrigerator according to this invention includes a compressor, a first high pressure on-off valve and a second high pressure on-off valve connected with a high pressure outlet port of the compressor, a first low pressure on-off valve and a second low pressure on-off valve connected with the low pressure inlet port of the compressor, a cryocooler comprising a regenerator, a cold head and a pulse tube arranged in series in line and connected with the first high pressure on-off valve and the first low pressure on-off valve in the regenerator, a cylinder member connected with the pulse tube at one end, and connected with the second high pressure on-off valve and the second low pressure on-off valve at the other end, a piston slidably disposed in the cylinder member and separating the interior of the cylinder into a first space connected with the inner space of the pulse tube and a second space connected with the second high pressure on-off valve and with the second low pressure on-off valve, a buffer space connected with the second space of the cylinder member and a buffer side on-off valve provided between the buffer space and the second space.
It is a preferable feature of the invention to provide the buffer space in the pulse tube refrigerator. This buffer space is connected with the second space connected with the second high pressure on-off valve and with the second low pressure on-off valve. The buffer side on-off valve is provided between the buffer space and the second space of the cylinder member. Accordingly, if the pressure condition in the second space is a low pressure condition, and the pressure in the second space increases to reach the same pressure as in the buffer by opening the buffer side on-off valve. If the pressure condition in the second space is a high pressure condition, the pressure in the second space decreases to reach the same pressure as in the buffer by opening the buffer side on-off valve. Provided that the pressure in the buffer space is an intermediate pressure between the high pressure (the pressure in the high pressure outlet port side of the compressor) and the low pressure (the pressure in the low pressure inlet port side of the compressor), the pressure in the second space becomes the intermediate pressure by opening the buffer side on-off valve. After the pressure in the second space reaches the intermediate pressure, the pressure in the second space becomes the high pressure by opening the second high pressure on-off valve, and the pressure in the second space becomes the low pressure by opening the second low pressure on-off valve. In this case, the pressure difference when the second high pressure on-off valve is open is the high pressure subtracted by the intermediate pressure. The pressure difference when the second low pressure on-off valve is open is the intermediate pressure minus the low pressure, whereas conventionally the second high pressure on-off valve is opened suddenly while the pressure in the second space is low, and the second low pressure on-off valve is opened suddenly while the pressure in the second space is high. Accordingly, the pressure difference, when the second high pressure on-off valve and the second low pressure on-off valve are open, is the high pressure minus the low pressure. Therefore, the pressure difference, when the second high pressure on-off valve and the second low pressure on-off valve are open according to the invention, can be decreased. Accordingly, the refrigeration efficiency has been improved by reducing the on-off valve loss.
It is another feature of the invention to provide a plurality of buffer spaces provided and connected with the second space having a plurality of buffer side on-off valves therebetween, respectively. Accordingly, the pressure in each buffer space can be supplied variably in accordance with the desired operation.
By controlling each buffer side on-off valve to increase or decrease the pressure in the second space gradually, and afterward by operating the second high pressure on-off valve or the second low pressure on-off valve to open, the pressure difference when the second high pressure on-off valve and the second-low pressure on-off valve are opened can be decreased. Accordingly, the refrigeration efficiency has been further improved by reducing the on-off valve loss.
It is another preferable feature of the invention to include the cryocooler including a first cryocooler comprising a first regenerator, a first cold head and a first pulse tube arranged in series in line, and the first regenerator being connected with the first high pressure on-off valve and the first low pressure on-off valve, and a second cryocooler comprising a second regenerator, a second cold head and a second pulse tube arranged in series in line, and the first cold head being connected with the second regenerator. Since the first cold head in the first cryocooler is connected with the second regenerator in the second cryocooler, the second cryocooler can utilize the refrigeration generated in the first cryocooler. Accordingly, in the second cryocooler, extremely low temperature of 4K (liquefied helium temperature), for instance, can be generated.
It is another feature of the invention to provide the piston separating the interior of the cylinder member into a first space connected with the inner space of the pulse tube and a second space connected with the second high pressure on-off valve and with the second low pressure on-off valve, and a third space connected with the second regenerator. Accordingly, the interior of the cylinder member is divided into three spaces. By providing the third space, the operation heat normally emitted as waste heat according to the movement of the piston can be returned to the regenerator side. Since the work of the compressor is decreased, the refrigeration efficiency can be improved.
It is still another feature of the invention to provide the piston elastically supported by an elastic element, such as a spring, in the cylinder. Accordingly, the piston can stably reciprocate within the cylinder. Rubber or another element can be substituted for the spring as an elastic element.
Any mechanical element, fluid element or magnetic element can be used as the elastic element for supporting the piston in the cylinder. An actuator may be adopted as the mechanical element. An operating gas in the cylinder may be adopted as the fluid element. As the magnetic element, a magnetic material piston and a coil wound around the cylinder may be used to energize the coil to generate electromagnetic induction for driving the piston.